The present invention relates to anti-aliasing filters for optical systems which sample the scene at a lower frequency than the frequency of the scene content, and to autofocusing devices for any imaging system, particularly those used in single lens reflex ("SLR") cameras.
In SLR cameras, it has become desirable to provide automatic focusing to reduce or eliminate manual focusing errors. One such system uses a "through-the-lens" autofocusing device. In this device, the central portion of the reflex mirror is partially transmitting, such that some of the light which would normally be imaged at the eyepiece, passes through the mirror, and is reflected by a second mirror to a focus sensing device at the camera base. This focusing device consists of a row of micro-lenses which bisect the image plane, a corresponding row of detector pairs beneath them, a charge-coupled device ("CCD") which samples the detector signals, and associated electronic circuitry to determine the position of best focus.
Each micro-lens projects an image of the camera lens exit pupil onto a pair of semicircular detectors, such that each detector "sees" a different portion of the exit pupil and scene. The row of detector pairs are electronically sampled such that all signals from "right-hand" detectors may be compared with the signals from corresponding "left-hand" detectors. When the camera is in focus, the signatures collected by the corresponding detector pairs are identical. If the camera is out-of-focus, the signatures are mismatched and displaced. The degree and direction of focus error is calculated from the signature displacement by a microcomputer in the base of the device.
These systems perform adequately, except when the spatial frequency of the scene content imaged on the detector pairs is higher than the spatial frequency of the detector pairs themselves. In this case the signatures collected by corresponding detector pairs may not match at best focus, resulting in aliasing, or the inability of the system to reach best focus.
Ideally, the problem of aliasing would be solved by placing a filter at the exit pupil of the camera lens to reduce the lens resolution to a value below the spatial frequency of the detectors, thus eliminating the high frequency scene content induced ambiguity. However, placing a filter at the exit pupil or between the focusing lens and the reflex mirror is impractical, since it would both distort the image and reduce the illumination of the film and eyepiece.
It is accordingly a primary object of the present invention to provide a system having an improved filter which eliminates aliasing problems in focusing devices which can be located in a position other than the lens exit pupil.